Hi,
The license enforcement will start as of January 15, 2026, but the exact date for your environment starts at the first anniversary or renewal date of your Dynamics 365 agreement. In case you have an anniversary or renewal on May 12, 2026, then this is your "T" date. 30 days prior to this date, unlicensed users will start getting in-app notifications. 15 days after this date, the technical validation will start, and users can then lose access to the application.
I'm not sure if I understand your comments about customizations. Custom objects and ISV solutions are not part of the license calculation at this moment in time. For these objects, you will not see any data on the Licenses usage summary page. They are initially considered to require a Team members license. In case a user has only one security role that consists of custom securable objects only, the User licenses will show 'None', but at a minimum, a user should have a license attached, so that will be the Team members license.
Here, there is a bit tricky part. In case you have a new menu item, data entity, or service operation based on an existing object, then multiplexing will play a role. E.g., you created a new menu item for the All customers page with a new name, and that will open the form with a different view. In this case, with write access, the calculation will not include this menu item, but the user actually does require the correct Finance or SCM license. Technically, the application will not block the users, but here you are still responsible for being compliant.
You can find some additional help in a FAQ document created by Microsoft, and blog posts on the internet. I also created some blogs that you can find here:
Security topics - Dynamicspedia. On this security landing page, there are blogs with updates and tips in the first section for User Security Governance. The last blogs contain also links to useful other content on my blog, but also from others and a link to the FAQ document.
I hope this helps you for better understanding. Feel free to ask more specific questions in case you still have some doubts.